Fiber optic communication networks are increasingly being deployed with rapid growth. Especially rapid is the growth of segments that carry multi-gigabit digital data on multiple wavelengths over a single fiber optic strand. The wavelength channel density and data rate carried on an individual wavelength continues to increase, especially for 100 G optical communication systems with multi-symbol modulation constellations and tight channel spacing. Both of these advancements, however, lead to an increase in nonlinear impairments like cross-talk. For passive optical fibers, the cross-talk mechanisms include cross-phase modulation (XPM), four-wave mixing, and Raman cross-talk.
These non-linear impairments, arising due to Kerr nonlinearity and the Raman effect, are additive to the overall interference level. The addition occurs in terms of each additional wavelength channel contributing a cross-talk component to the overall interference level. It is well known to those of ordinary skill in the art that the details of the bit pattern on each channel are important to the accurate estimation of the noise levels. The additive effect also occurs in optical communication systems that have multiple optical spans with intermediate optical amplification, such that each optical span additively contributes a cross-talk component to the overall noise level. The additive property of cross-talk implies that, whenever there is signal correlation, the interference level will be maximized. At the same time, signal anti-correlation may be used to minimize the interference level.
Optical phase conjugation (OPC) has been considered by many for the purpose of combating dispersive broadening. Early work considered only the linear dispersive signal distortion, which was compensated for by positioning the OPC mechanism in the center of the optical fiber link. Subsequent work considered that intra-channel signal distortion, such as self-phase modulation (SPM), induced by the Kerr effect in an optical fiber, may also be compensated for by positioning the OPC mechanism in the center of the optical fiber link. Such simultaneous compensation of dispersion and SPM places simultaneous constraints on the approximate equality of both optical fiber transmission dispersion and accumulated nonlinear phase shift on the opposite sides of the optical fiber link. This has been considered in conjunction with applications for single wavelength channel systems, for example. Other work has considered the benefits of OPC for single channel performance improvements with simultaneous dispersion compensation. Still other work has considered a specific implementation of OPC that introduces its own dispersion and ways to mitigate the dispersion with a corresponding dispersion compensation module (DCM).
Extensions to wavelength-division multiplexed (WDM) optical communication systems have also explicitly concentrated on the application of OPC to mitigating dispersion and four-wave mixing, independently and collectively. Stimulated Raman scattering effects on WDM optical communication systems, and the use of OPC for associated mitigation, has been considered, but under the assumption of unmodulated, and, hence, undispersed, optical carriers with a single input and single output optical fiber link.
Commonly assigned U.S. Pat. No. 7,310,318 (Dec. 18, 2007), discloses the use of OPC in conjunction with dispersion compensation for the express purpose of overcoming optical communication system impairments induced by Kerr effect nonlinearity in multi-channel WDM optical communication systems with multiple optically-amplified optical fiber spans. Both intra-channel SPM and inter-channel XPM accumulated over multiple spans may thus be mitigated. Further, extensions to the multi-channel WDM optical communication systems must account for the inherent gradual difference in dispersion presented by the optical transmission fiber to the individual channels as they change from short to long wavelengths. This dispersion slope across the wavelength range occupied by the channels significantly impairs the effectiveness of the impairment mitigation. Further, the presence of reconfigurable optical add/drop multiplexers (ROADMs) and complex channel traffic patterns reduces the mitigation effectiveness.
Thus, the current state of the art considers OPC for point-to-point optical links. There are two primary reasons for this: (1) OPC nonlinear XPM cancellation only works acceptably if near-neighbor channels have the same start and end points, such that the OPC mechanism may be positioned approximately half-way through the optical communication system to provide for effective nonlinear cancellation and (2) OPC flips channel wavelengths around the central spectrum pump (for example, a 40-channel optical communication system with an OPC pump located at channel 20 flips channels 19-21, 18-22, 17-23, etc.)—thus, managing channel assignments is very difficult in optical communication systems with many ROADMs.